Male mice of the NIH BALB/C and Swiss Webster strains were made aggressive by housing them individually for 3 weeks and were used as attackers (trained fighters). Other males of the NIH BALB/C and C57BR/cdJ (Jackson Laboratories, Bar Harbor, Maine) strains, housed in groups or individually, were subjected to daily 10-minute exposure to the trained fighters for up to 2 weeks. The victims were sacrificed and their adrenal glands assayed for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase activities. Catecholamines (CA) were determined in arterial blood, adrenal glands and discrete areas of the brain. Three days of exposure to attack did not induce any changes, whereas after one week there were striking increases in adrenal enzyme activities. After 2 weeks of repeated exposure to attack, CA increased in adrenal glands and blood of both victim strains, but brain TH and CA levels remained unchanged. We will ascertain the time course of the return of changes to control levels by interposing various periods of rest after one or two weeks of daily attack. We also intend to use various classes of psychoactive drugs in order to alter and/or reverse the biochemical changes, or to prevent their occurrence.